This invention relates to hydraulic control valve assemblies for vehicle hydraulic braking systems, for example for braking systems of the electro-hydraulic type.
Known hydraulic control valve assemblies, for example solenoid-control valve assemblies for use in electro-hydraulic vehicle braking systems, typically incorporate a spool working in a bore in which movement of the spool acts to connect and disconnect passages for providing communication between an hydraulic power source, brake, and an hydraulic master cylinder. Such spools and the respective housings provided with the bores in which the spools work have to be manufactured to high accuracy and very small tolerances in order to ensure that any leakage around the spool or across edges of the spool is reduced to an absolute minimum. Since the clearance between the spool and the housing is very small, care must be taken to avoid or eliminate any imperfections or distortions in the bore which could cause the spool to stick or lock at least during attempted movement within the housing. Since the spool housing has to be secured within a body in a fluid tight chamber, it is usual to clamp a housing by using a plate, or the body of the solenoid, up against the bottom of the bore within the body in which the housing is received. The application of a clamp load can, if sufficiently large, cause distortion of the housing bore to a degree sufficient to prevent the spool, itself, from moving freely within the bore in the housing. As the effective length of the housing, and the depth of the bore within the body and in which the housing is received, can vary from one assembly to another so can the clamp load since the extent of the clamp load depends upon the interference presented by the housing with respect to the clamp plate or the body.